ruby on rails - gocardless webhook signature mismatch -

i going through gocardless getting started guide, when try setup webhook (sandbox mode), don't correct value.

the code copy-pasted https://developer.gocardless.com/getting-started/api/staying-up-to-date-with-webhooks/#building-a-webhook-handler

and looks this

computed_signature = openssl::hmac.hexdigest(openssl::digest.new('sha256'),                                              secret,                                              request.raw_post)  provided_signature = request.headers['webhook-signature']  rack::utils.secure_compare(provided_signature, computed_signature)  # => false 

what missing? help